The present invention relates to a process for measuring the presence and various qualities of fluids, and materials containing fluids. More specifically, the present invention describes a process for detecting minute compositional changes in single sampling or continuous flow monitoring of fluids which offers extreme sensitivity, simplified temperature compensation, probe design, materials and control electronics.
A myriad of fluids are used in many scientific and industrial processes, as well as in end user applications. Initial, in-process and in-use testing of these fluids can often help prevent potential problems. Many processes rely on precise mixtures of fluids, slurries, suspensions or wetted materials and require accurate feedback on the resultant mixtures. End users often depend on accurate compositions of fluids, slurries, suspensions or wetted materials for safe and efficient use. Qualitative measurement of these materials can often prevent costly mistakes, damage or injury.
Electronic analysis of fluid compositions has historically been complicated by the fact that generally any such fluid has a dielectric constant, conductance and double-layer effects, each of which produces complex electrical responses. While measurements of these qualities are commonplace, they are plagued with instrumental difficulties such as probe design, erratic temperature dependencies and complex control electronics in the effort to get accurate and sensitive results.
In-use or in-process controls often require sensors capable of properly handling varying levels of flow, pressure and temperature while accurately measuring compositional changes. Current methods of measuring the dielectric constant or conductance of a fluid require either a very small range of variance in any of these effects, or extreme and technically complex compensations for them.
The dielectric constant of fluids is a common qualitative measure associated with fluids. It is known that the dielectric constant in solids is a measure of the ability of molecules to polarize or shift their internal charges in response to external fields. In fluids, the molecules Are also able to move about, rotating to orient in a field and/or migrating within the fluid. In electronic terms, the dielectric constant is the analog of a capacitor.
Many patents exist that are directed to measuring the capacitance of fluids. U.S. Pat. Nos. 4,132,944, 5,497,753 and 5,507,178 are representative of capacitance-measuring techniques.
Conductivity (the reciprocal of electrical resistance) is another common measure used to produce a qualitative indication of fluid compositions and charged species in a fluid. Charged species, or ions, present in a fluid provide a means for the passage of electrons through a fluid. The more ions present, the lower the electrical resistance of the fluid and the greater the magnitude of current that can flow through the fluid. In electronic terms this phenomenon is the analog of a resistance.
Numerous patents have been directed to fluid conductivity measurements, including U.S. Pat. Nos. 4,132,944, 4,634,982, 6,169,394 and 6,232,783, all representative of conductivity based applications.
Both of the above-described measures are greatly affected by temperature and other influences. In many cases, the precise theory behind these wide variances is not directly known or reliably predicted and varies considerably dependent on composition.
Measurements of conductivity and dielectric properties together have been performed in the past in efforts to simplify and solve many of the problems highlighted above. U.S. Pat. Nos. 4,516,077 and 6,169,394 are representative of this approach. In the latter patent, complex measurements were made of the electrical impedance of a fluid (i.e., the effect of a parallel resistance and capacitance). Unfortunately, this invention used complex electronics in generating a wide range of excitation frequencies, while-variances such as temperature dependencies were not addressed.
In U.S. Pat. No. 4,516,077, a sensor is described which is useful in a limited number of solvent solutions including water, alcohols and glycols. This invention included a method of electronically charging a fluid, disconnecting the charging means, and then measuring the time necessary for the charge across the fluid to dissipate (termed the xe2x80x9cintrinsic time constantxe2x80x9d). This invention essentially measures the re-diffusion rate of the polarization and electrical charges as they return to equilibrium devoid of any external electrical influences and is greatly affected by temperature and fluid flow rates.
The measurement of any fluid quality is complicated by the electrode-fluid interface. Each such interface includes its own resistance and capacitance, which are known to often be larger than those of the fluid itself. Electrochemical reactions caused by the introduction of an electrical current into a fluid can cause electrode corrosion and contamination. Sensed voltages or currents often need amplification and signal conditioning to provide suitable readings. These, and other problems, have seldom been addressed in previous inventions.
Therefore, it is desirable to develop an invention that uses the electrical qualities of the fluid to provide the primary measure while avoiding the above-described complications.
The present invention relates to a process for measuring the presence and various qualities of fluids and materials containing fluids. It offers improved performance over previous methods in its range and sensitivity, as well as relative insensitivity to temperature and fluid flow. In addition, this process offers simplified design and measurement.
The present invention includes a process and an apparatus for controlling and measuring various electrochemical effects of simplified electrochemical cells. However, the underlying effects measured are complex in nature. The present invention controls some of the individual influences of those effects to derive a measurement that has advantages over previous techniques and is termed here Transient Immitivity Response (TIR).
The primary feature of the invention is the use of a capacitance external to the cell to accumulate, control and limit the electrical currents passing through the cell. Transient immitivity response refers to the interactions between this capacitance and the current transfer mechanisms within the electrochemical cell. These interactions create a complex rate of electrical charging and discharging of this external capacitance that can be measured in many different ways. This capacitance, the cell configuration and other external components may be adjusted to enhance or reduce the effect of various charge transfer mechanisms and to fit the invention to virtually any fluid. The transient immitivity response is the time related complex rate at which charge is passed through the cell and accumulated on the external capacitance.
One embodiment according to the invention includes two electrodes spaced apart from each other and both in contact with a fluid-being tested. This embodiment includes an excitation source for providing a time-varying excitation voltage to a first one of the electrodes. The excitation voltage is switched between a first defined voltage level and a distinct second defined voltage level. The first and second voltage levels are alternatively applied to the first electrode for specific time periods. This source has a low source resistance such that it is able to supply sufficient electrical current to change the first electrode""s electrical potential in a minimal time and thereby rapidly charge the first electrode""s capacitance.
According to the invention, a defined capacitance is located between the second electrode and an electrical or circuit ground. The ground has a defined voltage. This embodiment also includes a voltage detector for detecting a sensed voltage induced on the defined capacitance. The sensed voltage is proportional to electrical charges conducted through the fluid from the first electrode to the second electrode as a consequence of the excitation voltage applied to the first electrode. This voltage detector has a very high resistance to electrical ground such that there is no substantial current flow through it from the cell. Examples of suitable voltage detectors include current generation FET transistors, op amps and CMOS logic circuits having input resistances greater then 1011 ohms.
In this embodiment, the voltage level at the excitation source is held constant at least until the cell is at equilibrium when a fluid is present. If there is no fluid present, no voltage will be detected at the sensing or detecting means. If a fluid is present at equilibrium, all portions of the electrochemical cell of the embodiment will be at essentially the same voltage as the excitation voltage and the voltage sensed at the second electrode will be essentially equal to the voltage at the first electrode. The excitation voltage of the first means is then switched to a second voltage level. The cell will now work to come to equilibrium at this second voltage level.
The embodiment further includes a means for determining one or more time intervals between the switch in first and second defined voltage levels and when a sensed voltage at the capacitance attains one or more selected voltage levels. These time intervals represent the transient immitivity response of the fluid. Alternately, this means may measure the voltage attained at the capacitance at one or more predetermined time intervals after the switch in first and second defined voltage levels. Once again, providing a measure of the xcex94voltage/xcex94time nature of the transient immitivity response. The voltage level attained at the second electrode is a time-related function of all of the resistances and capacitances of the electrode interface and the fluid, and the change in voltage of the first excitation source. This embodiment is further capable of providing the transient immitivity response as a digital or analog output. A lack of a changing sensed voltage may indicate a lack of fluid between the electrodes. While this single time, or rate, measurement embodies the basis for the present invention, two or more measurements of the time-related response of this electrochemical cell system may be used to elucidate more subtle information.
The present invention is also a method of using an apparatus to obtain a transient immitivity response of a fluid. Initially, first and second electrodes are selected and the electrodes, spaced apart from each other, are brought into contact with a fluid. Time varying excitation voltage is then applied to the first electrode. The excitation voltage is subsequently switched between a first defined voltage level and a distinct second defined voltage level so that the first and second defined levels are alternately applied to the first electrode for specific time periods. The excitation source is further characterized by having a low resistance in order for a minimal switch time to exist when the excitation voltage is switched between the first and second defined voltage levels.
The method further includes providing a defined capacitance between the second electrode and an electrical or circuit ground. The ground has a defined voltage. A sense voltage is then detected as having been induced on the capacitance, the sense voltage being proportional to electrical charges conducted through the fluid from the first electrode to the second electrode as a consequence of the excitation voltage applied to the first electrode. The detector used is preferably characterized by having a high input resistance to minimize external current flows.
Following detection of a sense voltage induced on the capacitances, one or more time intervals are determined between the switch between first and second defined voltage levels and when the sense voltage at the second electrode attains one or more specific voltage levels. Alternately, one or more voltage levels attained at predetermined time intervals from the time the excitation voltage is switched between a first and second defined voltage level may be measured. These time intervals and voltage levels represent the transient immitivity response of the fluid and may be subsequently provided as digital or analog output.
It is known that the above-described resistances and capacitances are themselves functions of the fluid under test, fluid flow, temperature, electric potentials and other effects. The particular combinations of these effects as measured by the present invention can produce reduced dependence on flow and potential as well as reducing the variance in temperature dependencies caused by fluid composition.
It is therefore an object of the present invention to provide a fluid sensor which overcomes many of the limitations of the prior art.
It is another object of the present invention to provide a sensor for the presence of a variety of fluids and fluid bearing materials.
It is a further object of the present invention to provide a sensor which can qualitatively measure the difference between various solvents and fluid compositions.
Various other features, objects, and advantages of the invention will be made apparent to those skilled in the art from the accompanying drawings and detailed description thereof.